This invention relates to a wide-angle lens system for use on single-lens reflex cameras.
A single-lens reflex camera has a mirror placed between the lens and the image plane, so the lens system must have a back focal distance longer than a certain value. To meet this need, a wide-angle lens system is usually of a retrofocus type having a longer back focal distance than the focal length. Retrofocus configuration comprises, in order from the object side, a front group having negative refractive power, a diaphragm and a rear group having positive refractive power. The asymmetry of refractive power with respect to the diaphragm contributes to increasing aberrations such as spherical aberration, coma, distortion and astigmatism. In order to correct these aberrations, more lens elements have to be used. However, with the retrofocus configuration, focusing is usually done by extending the overall lens system, so the use of more lens elements increases the weight of the lens groups to be moved and this is unfavorable for the purpose of autofocusing. This weight problem of the focusing lens groups is extremely important with wide-angle lens systems for use in medium-format cameras using Brownies and large-format cameras.
From an autofocusing viewpoint, it is preferred to adopt a lens system that effects focusing by moving only part of the lens groups and those which are of particular advantage are an inner focus and a rear focus system that remain constant in overall length during focusing. However, compared to the type that extends the whole lens groups, the rear focus system suffers from increased aberrational variations during focusing and this phenomenon is particularly noticeable if the rear focus system is used with the retrofocus configuration.
An object, therefore, of the invention is to provide a three-group, inner-focus, wide-angle lens system of retrofocus configuration that has a half viewing angle of about 44xc2x0 and an F number of about 3.5.
This object of the invention can be attained by a wide-angle lens system comprising, in order from the object side, a negative first lens group, a positive second lens group, and a positive third lens group having a diaphragm, characterized in that the second lens group moves along the optical axis during focusing and that said lens system satisfies the following conditions (1) and (2):
0.5 less than |f1/f| less than 1.2xe2x80x83xe2x80x83(1) 
0.7 less than f2/f less than 1.8xe2x80x83xe2x80x83(2) 
where f is the focal length of the overall system at focusing at infinity, f1 is the focal length of the first lens group, and f2 is the focal length of the second lens group.
Preferably, the wide-angle lens system of the invention further satisfies the following condition (3):
2.0 less than f3/f less than 7.0xe2x80x83xe2x80x83(3) 
where f3 is the focal length of the third lens group.
In another preferred embodiment, the wide-angle lens system of the invention further satisfies the following condition (4) in addition to the conditions (1)-(3) or the conditions (1) and (2):
1.1 less than mxc2x7f/xcex94X2 less than 2.0xe2x80x83xe2x80x83(4) 
where m is the magnification of the overall system at the closest focusing distance, and xcex94X2 is the total amount of movement the second lens group makes during focusing from infinity to the closest distance.
In yet another preferred embodiment, the wide-angle lens system of the invention further satisfies the following condition (5) in addition to the conditions (1)-(4) or the conditions (1)-(3) or the conditions (1) and (2):
1.67 less than TL12/TL3 less than 2.8xe2x80x83xe2x80x83(5) 
where TL12 is the distance, at focusing at infinity, between the surface of the first lens group which is the closest to the object and the surface of the second lens group which is the closest to the image, and TL3 is the distance between the surface of the third lens group which is the closest to the object and the surface which is the closest to the image.
The present disclosure relates to subject matter contained in Japanese patent application No. 2000-37785 (filed on Feb. 16, 2000), which is expressly incorporated herein by reference in its entirety.